Split the main up loop into two threads.

The reader and the writer now communicate through a channel and the packet
cache.  If the writer thread drops behind, we drop packets after inserting in
the packet cache, which avoids building a backlog.

client.go

@@ -372,7 +372,7 @@ func addUpConn(c *client, id string) (*upConnection, error) {
 			}
 		}
 
-		go upLoop(conn, track)
+		go readLoop(conn, track)
 
 		go rtcpUpListener(conn, track, receiver)
 	})
@@ -380,18 +380,19 @@ func addUpConn(c *client, id string) (*upConnection, error) {
 	return conn, nil
 }
 
-func upLoop(conn *upConnection, track *upTrack) {
+type packetIndex struct {
+	seqno uint16
+	index uint16
+}
+
+func readLoop(conn *upConnection, track *upTrack) {
+	ch := make(chan packetIndex, 32)
+	defer close(ch)
+	go writeLoop(conn, track, ch)
+
 	buf := make([]byte, packetcache.BufSize)
 	var packet rtp.Packet
-	var local []*downTrack
-	var localTime uint64
 	for {
-		now := mono.Microseconds()
-		if now < localTime || now > localTime+500000 {
-			local = track.getLocal()
-			localTime = now
-		}
-
 		bytes, err := track.track.Read(buf)
 		if err != nil {
 			if err != io.EOF {
@@ -409,7 +410,8 @@ func upLoop(conn *upConnection, track *upTrack) {
 
 		track.jitter.Accumulate(packet.Timestamp)
 
-		first, _ := track.cache.Store(packet.SequenceNumber, buf[:bytes])
+		first, index :=
+			track.cache.Store(packet.SequenceNumber, buf[:bytes])
 		if packet.SequenceNumber-first > 24 {
 			found, first, bitmap := track.cache.BitmapGet()
 			if found {
@@ -420,6 +422,45 @@ func upLoop(conn *upConnection, track *upTrack) {
 			}
 		}
 
+		select {
+		case ch <- packetIndex{packet.SequenceNumber, index}:
+		default:
+			// The writer is congested.  Drop the packet, and
+			// leave it to NACK recovery if possible.
+		}
+	}
+}
+
+func writeLoop(conn *upConnection, track *upTrack, ch <-chan packetIndex) {
+	var localTime uint64
+	var local []*downTrack
+
+	buf := make([]byte, packetcache.BufSize)
+	var packet rtp.Packet
+
+	for {
+		now := mono.Microseconds()
+		if now < localTime || now > localTime+500000 {
+			local = track.getLocal()
+			localTime = now
+		}
+
+		pi, ok := <-ch
+		if !ok {
+			return
+		}
+
+		bytes := track.cache.GetAt(pi.seqno, pi.index, buf)
+		if bytes == 0 {
+			continue
+		}
+
+		err := packet.Unmarshal(buf[:bytes])
+		if err != nil {
+			log.Printf("%v", err)
+			continue
+		}
+
 		for _, l := range local {
 			err := l.track.WriteRTP(&packet)
 			if err != nil && err != io.ErrClosedPipe {